A key switch is capable of generating a voltage signal when an instant force is applied to the switch. The resultant voltage signal may be employed in applications involving computers, alphanumeric keyboards, kitchen appliances, intrusion detectors and numerous other electronic devices.
Conventional metal contact key switches become unreliable after extended periods of usage. For example, contact resistance tends to increase with age, and poor contact at the contact points produce undesirable chattering and/or bounce. In an effort to overcome the general unreliability of metal contact key switches, Hall effect key switches employing magnetic resistance elements, or capacitive key switches have been developed, each requiring elaborate or complicated parts and/or controlling circuits.
More recently, poled polymeric piezoelectric films functioning as the pressure sensing component have been developed, which films substantially overcome the deficiencies of metal contact key switches, as well as the deficiencies inherent in piezoelectric ceramics, which are hard, brittle, easily broken, difficult to machine into complex shapes, and often generate spurious voltage signals from sound waves bouncing off their surfaces.
Generally, polymeric materials are non-piezoelectric. Polyvinylidene fluoride (PVDF) however, may be made piezoelectric. PVDF is approximately 50% crystalline and 50% amorphous. The principal crystalline forms of PVDF are the highly polar .beta. form and the non-polar .alpha. form. High piezo response is associated with the polar .beta. form. By carefully controlling process steps to polarize the film, including mechanical orientation and treatment in an intense electric field, a highly piezoelectric and pyroelectric film results. Such a film is commercially available under the trademark KYNAR.RTM., a product of Pennwalt Corporation, Philadelphia, PA., assignee of the present invention.
The film is flexible, tough, and inexpensive, possesses a low modulus, and a low mechanical Q factor, and hence, little or no chattering. Many present day keyboards employ PVDF films as the piezoelectric sensing component therein. The invention is not limited to films made of PVDF only, and copolymers of vinylidene fluoride, and copolymerizable comonomers such as tetrafluoroethylene and trifluoroethylene, for example, may be employed.
In U.S. Pat. No. 4,234,813, split key keyboards are disclosed. The signal generating row and column signal areas are disposed on opposite sides of a PVDF film, for example, as well as the earth or ground electrodes. Cross-talk is eliminated by such design. A disadvantage of split key keyboards resides in its inability to completely eliminate cross-talk on keyboards involving a rather large number of key switches.
In British Pat. No. 1,550,691, a laminate of PVDF films, for example, for use in keyboard transducers, includes ground electrodes in face-to-face relationship to substantially eliminate cross-talk. The keyboard however is not resistant to EMI.
In FIG. 1 of U.S. Pat. No. 4,328,441, a structure is disclosed similar to an embodiment of a structure disclosed by applicant. The electrodes 14 and 18 however, are nickel or aluminum, for example, which are generally incapable of being strained indefinitely for switch application purposes.
In FIG. 11 of U.S. Pat. No. 3,940,637, a pair of poled PVDF films has electrodes formed on one face thereof with ground electrodes formed on the other faces. The films are piled one on top of the other with an electrically insulating film separating the PVDF films. The resultant composite is not EMI resistant nor are the electrodes believed capable of permitting indefinite straining of the piezoelectric sensors.
The present invention discloses apparatus and processes wherein the signal generating electrodes and lead lines thereto are preferably silk-screened onto a surface of a pair of poled piezoelectric PVDF films. The ground electrodes are silk-screened onto the other surface of each of the pair of films, either completely thereover, or localized to register with the signal generating electrodes on the opposing side thereof, but slightly larger in order to eliminate EMI.
The signal generating electrodes will be disposed in face-to-face relationship with an electrically insulating layer therebetween. The resultant composite may be deflected and strained millions of times without failure, chattering, or interference from electromagnetic radiation.